Ceramic ion transport membranes are ceramic devices in the form of tubes, flat plates or honeycomb structures that are capable of oxygen ion transport or proton transport at elevated temperatures. Membranes may be formed from materials that are either mixed conductors that conduct both ions and electrons or ionic conductors that conduct ions only. Mixed conducting materials are used in various reactors that involve oxygen or hydrogen separation from a feed. Ionic conductors are also used in oxygen separation devices and in addition, solid oxide fuel cells designed to produce electrical power.
For instance, in case of mixed conductors that are used in forming oxygen transport membranes, oxygen ionizes at a cathode surface of the membrane to form oxygen ions. The oxygen ions are transported through the membrane where they emerge from an opposite, anode side to recombine into elemental oxygen. In such recombination, electrons are given up to the membrane and are transported to the opposite cathode side where they serve to ionize the oxygen. Such a membrane can be used in an oxygen generator and also, a chemical reactor designed, for example, to produce synthesis gases.
In ceramic membranes that are formed from ionic conductors, since the membrane is only capable of transporting the ions, an external circuit or conductive phase is provided for transport of the electrons. An external circuit is used in oxygen generators to apply an electric current across the membrane. In solid oxide fuel cells, the external circuit is used to conduct the electrons to a load. Conductive phases are used when the membrane is to serve in a pressure driven application.
It has long been recognized in the art that an ion transport membrane should be as thin as possible to minimize the internal resistance to the ionic transport. Thus, ion transport membrane are constructed as composite structures having a very thin dense layer, that is a layer that is air tight, formed from a material capable of conducting the ions and that is supported by one or more porous layers. The porous support layers can be inert or active and in fact, can be formed of the same material as the dense layer. The porosity allows for the separated oxygen to freely permeate through the support. At the same time, in case of active supports, there exists more area in which oxygen ions can recombine combine to form elemental oxygen and to give up electrons.
An example of a membrane incorporating a mixed conducing dense layer can be found in U.S. Pat. No. 5,240,480 in which a dense layer of a mixed conducting oxide is supported by one or more porous layers. Where multiple porous layers are used, the thickness of the layers and the pore size thereof successively increases from layer to layer.
U.S. Pat. No. 4,957,673 discloses a multilayer ceramic oxide structure for use in fuel cells or electrolysis cells in which layers of strontium lanthanum manganite sandwich a layer of yttria stabilized zirconia. The yttria stabilized zirconia is an ionic conductor to act as the electrolyte and the strontium lanthanum manganite serves as electrodes.
There are a variety of different ways of forming a composite ion transport membrane. For instance, the dense layer can be applied to a porous substrate by tape casting. Another common method is to tape cast layers of the composite. In this regard, in U.S. Pat. No. 4,957,673, discussed above, a composite tape is made having electrolyte and conductor layers that is pressed to laminate the layers together and then fired to form the finished article. A yet further method is to isopress ceramic powder mixtures, some of which contain pore formers such as disclosed in WO0224997 or to isopress a combination of tape cast film and ceramic powders containing pore formers such as disclosed in WO0224437. The green form resulting from tape casting or isopressing is heated to burn off the binder and any pore forming material. The heating is continued until the ceramic sinters to produce the finished article.
The problem with such methods of forming the green article is that it becomes difficult to form a thin dense layer that is free of defects. It has been found that the green form, as hereinabove described, will produce an unacceptable level of pinhole defects within the dense layer due to the evolution of gases produced during the burnout of the binder and pore forming material. Such defects are especially exacerbated in supports that have a large pore size. Thus, while it is possible to form a dense layer, such dense layer if too thin will have an unacceptable level of defects. If the dense layer is made sufficiently thick to reduce the occurrence of defects, its resistance to ionic transport will increase to unacceptable levels.
As will be discussed, the present invention provides a method of manufacturing a green form that is useful in producing a composite, ceramic ion transport membrane in which the degree of pinhole defects within the dense layer is reduced over the prior art to enable composite structures to be produced having a very low resistance to ionic transport.